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Up till now, we have learned both DFS and BFS in graph search. From the last chapter, we have seen how the backtracking method utilized DFS to  solve enumeration and CSP based problems. The type of problem we should solve with backtracking is the one that requires enumeration all possibilities. We have seen that most of the time, backtracking comes with high complexity. If the problem only requires us to find if 
\section{Bidirectional BFS}
\url{http://theoryofprogramming.com/2018/01/21/bidirectional-search/}
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